1. Field of the invention
The present invention relates, in general, to a method for fabricating a phase shift mask (hereinafter referred to as "PSM") and, more particularly, to taking advantage of inorganic resist, designed to be useful for alternate types, having superior light-screening effect.
2. Description of the Prior Art
The Most widely used phase shift masks have been based on the alternate type PSM. In such PSM the chrome screen pattern basically alternates with the shifter material.
In order to better understand the background of the invention, a description will be given of conventional representative methods for fabricating alternate type PSM, in conjunction with some drawings.
First, referring to FIG. 1, one conventional method for fabricating an alternate type PSM is shown.
As shown in FIG. 1 A, a chrome screen is formed on a mask substrate 1 and then, etched into screen patterns 4 which are each spaced apart. On the resulting substrate 1 a blanket of resist 2 is deposited which is then locally opened to selectively expose a region on which a shifter will be formed. Thereafter, an oxide film 3 for shifter, for example, SiO.sub.2 film is deposited entirely over the patterns.
As shown in FIG. 1B, all the oxide film 3 and the resist 2 but shifter pattern 3' are removed by a lift-off method, to complete the fabricating process of phase shift mask.
This method has a disadvantage that, when carrying out the lift-off process, particles may remain on the substrate, leading to contamination. In addition, uniform phase-shifting effect on the substrate cannot be accomplished owing to the topology of the shifter.
Referring to FIG. 2, there is shown another conventional method for fabricating an alternate type PSM.
First, as shown in FIG. 2A, a chrome screen is deposited on a mask substrate 1 and then, etched into screen patterns 4 which each are regularly spaced apart. A blanket of polymethylmethacrylate (hereinafter referred to as "PMMA") 5, an organic polymer, is deposited over the substrate and then, selectively exposed to E. beam as indicated by the arrow.
FIG. 2B shows the completion of phase shift mask. For this, the exposed PMMA is etched to form a shifter pattern 5' as shown in FIG. 2B.
Like the conventional method previously mentioned, this method cannot bring about a complete shifter effect owing to the topology of the chrome.
Besides, the above-mentioned conventional methods both have a significant disadvantage of occurrence of mask distortion resulting from stress on the substrate which is attributed to a thermal treatment according to the chrome deposition necessary for the screen or shifter.